Calcium and magnesium oxide and hydroxide couplers have been used in the preparation of sheet and bulk molding compound (SMC and BMC) in order to give compounders and molders of unsaturated polyester resin wide latitude in the molding of products. They permitted compounders and formulators to form a resin in bulk which was tack free and could be easily handled for forming a final product. The use of couplers also permitted the formation of reinforced resin compositions which essentially eliminated resin rich molding. Resin rich molding occurred with non-thickened compounds in that the polyester resin and vinyl monomer separated from the reinforcing medium e.g., chopped fiberglass.
A number of organometallic promotors and catalysts have been used for promoting the cure of polyesters. Although they were effective for enhancing the cure rate of neat polyester resins, they were suited for the preparation of sheet and bulk molding compound because they were too active at temperatures below the molding temperature and caused gelation prior to molding. Those that were acceptable in terms of shelf life were found to be ineffective for accelerating the cure at molding temperatures.